First off let me start by saying this is a theory and that my purpose is to try and have the main idea predicted. With that said not all of the things that will be said will be true. Therefore there will be many spoilers in both 8 Mile and Straight Outta Compton. With all that said lets get right into it.
Do you guys remember the ending of 8 Mile? Well if not heres a reminder: Eminem wins the rap battle against Papa Doc and becomes champion, but this rap battle competition happens every week so in turn this isn't a really big accomplishment . When i first watched the movie this had me thinking whats going to happen to him next, and thats when a couple of years later straight outta compton came out. After I watched the movie I had reflected about the ending quite a bit. This ending was when Dr. Dre was leaving death row, and Suge Night asked him what he was going to call his company (record label). Dr. Dre replied saying, "AFTERMATH". I wondered what happened when Eminem had walked off at the end of 8mile, and when I heard they were making a sequel to Straight Outta Compton I knew that 8mile would have to do with it.
My theory is that, after 8 Mile Eminem had walked off and tried to find another record label to sign with instead of being stuck in Detroit. He was successful in doing so as one of the people in the crowd of one of Eminem's rap battle was affiliated with Dr. Dre. Eminem will sign with aftermath and this will be introduced in the next movie. Along with this the next movie will introduce how Eminem was signed to Aftermath as well as putting the main spotlight on Eminem.
So in a way the Straight Outta Compton movie had set up the Eminem movie biography as well as incorporating 8mile as a prequel. Think of it this way: if Straight Outta Compton made so much money, why not make more movies including one of the most loved rappers of all time (Eminem).